Production of sintered slender shapes



13, 1964 s. E. TARKAN ETAL 3,145,100

PRODUCTION OF SINTERED SLENDER SHAPES Filed Dec. 21, 1962 FORM ND/N WHEEL GRINDER CROSS INVENTORS .S' TUART E.

A T TORNE Y United States Patent 3,145,100 PRODUCTION OF SINTERED SLENDER SHAPES Stuart E. Tarkan, Monsey, and Edward G. Wojtowicz,

Lake Secor, N.Y., assignors to Chromalloy Corporation, West Nyack, N.Y., a corporation of New York Filed Dec. 21, 1962, Ser. No. 246,415 13 Claims. (Cl. 75200) This invention relates to a method for inhibiting warpmetallic constituents by compressing a powder mixture.

in a mold to produce a desired shape and sintering the shape at an elevated temperature in a substantially nonoxidizing atmosphere, whereby to cause the compressed particles to sinter together into a dense structure. In situations where a powder compact has a marked tendency to shrink during sintering, warpage generally occurs with certain shapes having a high length to cross section ratio,

such as rods having a circular or other substantially regular cross section.

This problem is particularly acute in the production of articles by the powder metallurgy technique of liquid phase sintering, for example, in the sintering of high melting point materials, such as refractory carbides, in the presence of a liquid phase matrix metal. For instance, in producing rods from titanium carbide powder sintered together with a steel matrix, it is diflicult to produce substantially straight rods having a length to cross section ratio of 5 to 1 or greater capable of being handled easily by a centerless grinder. Generally, the rod would tend to warp out of shape during sintering sufliciently to render subsequent processing steps extremely, difiicult.

A method has now been discovered whereby long slender shapes can be produced by powder metallurgy in which warpage due to sintering is maintained at a minimum.

It is an object of this invention to provide a method for inhibiting warpage in the powder metallurgy produc tion of long slender shapes.

Another object is to provide a method of producing substantially straight sintered articles of manufacture having a high length to cross section ratio and having a general propensity to shrink during sintering.

The invention also provides as a unitary assembly a plurality of compressed slender shapes lying adjacent each other in parallel arrangement, said slender shapes being joined and supportably connected together via a thin breakable longitudinal web between each shape.

These and other objects will more clearly appear from the following description when taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is illustrative of a sintered rod which can be produced with my invention showing the relation of length to cross sectional thickness;

FIG. 2 shows in cross section a plurality of 'rods connected to each other by longitudinal webs and being supported at the web by triangular shaped ceramic supports;

FIG. 3 depicts in three dimensions a unitary assembly comprising a plurality of rodscompressed from powdered ingredients connected together longitudinally by' webs prior to sintering; i i V 7 'not exceed about 2: 1.

3,145,100 Patented Aug. 18, 1964 FIG. 4 shows in front view an enlargement of two rods connected together by a web and illustrating the ratio of cross sectional thickness to web thickness;

FIG. 5 illustrates the invention as applied to the production of rods with a square cross section;

FIG. 5a shows rods of rectangular cross section; and

FIG. 6 depicts one method for producing a unitary assembly comprising a plurality of web-connected rods by form grinding a green slab of compressed powder.

Stating it broadly, our invention is directed to a method for inhibiting warpage in the production of longitudinal slender shapes from powdered ingredients by compressing said ingredients into slender shapes followed by sintering said shapes at an elevated temperature which comprises, forming as a unitary assembly a plurality of compressed slender shapes lying parallel and web-connected 'toeach other via an alternating series of thin breakable webs, sintering the unitary assembly at an elevated temperature under substantially non-oxidizing conditions sufficient to form a dense structure, cooling said sintered assembly, and separating the sintered shapes from each other by severing, e.g., breaking, the connecting webs along their lengths.

We find that with the foregoing method, we can produce slender rods from powdered material by sintering with warpage greatly minimized. For example, we may produce slender cylindrical rods ranging in diameter from as low as one-eighth of an inch to as high as one inch and higher.

By slender rods, We mean rods whose ratio of length to cross-sectional thickness is preferably at least about 5:1 and higher. By cross-sectional thickness, we mean the thickness of a cross section which is substantially of uniform geometric shape. Such cross-sectional shapes may be in the form of a circle, a square, an equilateral or an isosceles triangle, regular polygons, or variations of the foregoing. For example, the cross section may be a rectangle whose width to thickness ratio does not exceed about 2:1; or an ellipse whose length to thickness likewise does not exceed about 2: 1; in other words, any cross section whose ratio of maximum Width to thickness does Thus, in the case of a rectangular cross section, the length of the rectangular rod should be at least 5 times greater than the thickness of the rectangle.

Referring to FIG. 1, a cylindrical rod 1 is shown having a diameter or thickness T and a length L. Such a rod, having an L to T ratio of 5:1 and higher, e.g., 10:1, when produced from compressed metal powders or a compressed mixture of metal powders with refractory carbides having a propensity to shrink tend to warp and bend out of shape. This is particularly true of small diameter rods of diameter or thickness below one-half inch and even below one-quarter inch.

We have found that such warpage can be greatly minimized by preparing as a unitary assembly. a plurality of rods in parallel arrangement interconnected by a thin web running longitudinally along each rod. 7 Because of the connecting webs, a rigidized structure is obtained which resists warping stresses.

Referring to FIG. 2, a'plurality of unsintered rods 2, 3, 4 and 5 are shown connected by webs 2a, 3a and 4a and preferably supported by longitudinal triangular magnesia supports 6 positioned along the webs preparatory to sintering. Because the rods are united as a unitary assembly, the assembly is sufficiently rigid to be supported off the furnace hearth during sintering Where it is desired to keep surface defects to a minimum which otherwise might arise by contact of the rod with the furnace hearth. How- 3 ver, such supports need not be used so long as the furnace hearth is clean. In producing the web, a reduced portion in the form of a neck is preferred to assist in breaking the web apart. This may take the form of a shallow V-groove as shown in FIG. 2.

FIG. 3 shows in three dimension a plurality of webconnected rods 7 to 11 connected by thin webs 7a to 10a :as shown.

The dimensional thickness of the web should be such as to enable separation of the rods by severing the webs along their length. Preferably, the web thickness should fall within the range of about 0.025 inch to about 0.075 inch, and more preferably, from about 0.03 to 0.06 inch, depending upon the thicknessof the cross section. Referring to FIG. 4, an enlarged front view is depicted showing a pair of circular rods 12 and 13 joined together by a web 14, the ratio of the cross sectional thickness or diameter of the rod T to the web thickness 1 being at least about 4:1. As shown in FIG. 4, the web is slightly necked down to aid in severing the two rods after completion of sintering. The ratio of the cross-sectional thickness to the web thickness may range from about 4:1 to 30: 1, preferably from about 4:1 to 20:1. Thus, in working over the web thickness ranges of either about 0.025 to 0.075 inch or about 0.03 to 0.06 inch, we may work within the thickness ratio range of either 4:1 to 30:1 or 4:1 to 20: 1. Likewise, the ratio of the length of the rod to the thickness of the cross section may range from about :1 to 50:1 or from 5:1 to 35:1.

The width of the web should preferably not exceed three times the thickness of the web and more preferably not exceed twice the thickness. In producing straight circular rods of about one-eighth inch diameter, we have found that a web thickness of about 0.03 inch and a web width of about 0.06 inch very satisfactory for my purposes. In producing straight circular rods of about three-quarter inch in diameter, we have found a web thickness of about 0.06 inch at a width of about 0.06 to be satisfactory.

FIG. 5 is illustrative of another geometric shape which can be produced by our invention. In this figure, the cross section of square rods 15 to 19 are shown interconnected by means of webs 15a to 18a. In FIG. 5a, rods 20 and 21 of rectangular cross section are shown connected together by web 20a, the rod having a cross sectional width W and a thickness T at a W to T ratio ranging up to about 2:1.

In producing a unitary assembly comprising a plurality of web-connected cylindrical rods, we prefer to start with a compacted rectangular slab of sufficient green strength produced from compressed metal powder and contouring the slab with a form tool to produce the desired shape prior to sintering. One method is shown in FIG. 6. A partially ground compressed slab 25 is shown supported on a chuck 26 against guide element 27 below a form grinding wheel 28 partially shown. Where the material shown is magnetic in nature, it may be magnetically held to the chuck. Non-magnetic materials may be held in place by mechanical means. The grinding wheel is contoured to produce two circular quadrants 29 and 30 corresponding to quadrants 29a and 30a form ground into the surface of compressed slab 25. The form the rods eventually take upon completion of the grinding is indicated phantomly by dotted lines. The chuck is adapted to travel crosswise of the wheel and is indexed for each position of the grind. After one-half side of the rods have been formed, the slab is turned over on the opposite side and the form grinding completed.

The unitary assembly resulting from the grinding operation is then subjected to an appropriate sintering temperature for a time sufficient to insure a dense structure and the sintered assembly then cooled to room temperature. The joined rods thus sintered come out straight and undistorted. After sintering, the rods are severed along each web and the web-flash removed from the side of each rod by hand snagging on a silicon carbide grinding wheel. Thereafter, they are ready for conventional centerless grinding.

The production of straight slender rods by powder metallurgy from a tool steel composition of high carbon content comprising titanium carbide and a steel matrix presents a problem as generally the ingredients are subjected to liquid phase sintering which leads to distortion of slender rods during sintering. We find that my invention is particularly suited for producing rods of the foregoing composition.

As one example, in producing rods to a finished size of one-eighth inch diameter and 4 /2 inches long, powdered titanium carbide is mixed with powdered steel-forming ingredients and the mix then pressed into a rectangular slab. In producing a composition containing 40% by Weight of TiC and 60% by weight of steel-forming ingredients, 1000 grams of TiC of about 5 to 7 microns in size are mixed with 1500 grams of steel-forming ingredients comprising carbonyl iron powder of 20 micron average size and 0.80% carbon by milling in a steel mill. The powdered ingredients contain 1 gram of parafiin wax for each grams of mix. The milling is conducted for about 40 hours, the mill being half full with stainless steel balls, using hexane as a vehicle.

After completion of the milling, the mix is removed and vacuum dried. A proportion of the mixed product weighing about grams to produce one-eighth inch diameter rods is compressed into a split die to form a slab approximately 2.15 inches wide by 5.2 inches long by 0.23 to 0.24 inch thick at a pressure of about 15 t.s.i. The slab is thereafter form ground as shown in FIG. 6 to form a unitary assembly of seven interconnected oversized one-eighth inch rods, making allowances for shrinkage and finish grinding tolerances. The oversize may generally range from 0.02 to 0.05 inch. The Web between the rods had a thickness of 0.03 inch and a Width of 0.06 inch.

The unitary rod assembly is then subjected to vacuum sintering, i.e., liquid phase sintering, at a temperature of about 1450 C. for about one half hour at a vacuum corresponding to 20 microns of mercury or better. After completion of sintering, the assembly is cooled and then annealed by heating to 900 C. for 2 hours followed by cooling at a rate of about 15 C./ hour to about 100 C. and thereafter furnace cooled to room temperature to produce an annealed micro-structure containing spheroidite.

The annealed slab is removed from the furnace, the rods broken apart, and then hand snagged on a silicon carbide grinding wheel to remove the remaining webflash and finally centerless ground to size. The snagged rods prior to finish grinding were substantially straight and undistorted.

In producing 4% inch rods of various diameters having the same composition as stated above from slabs or blocks 2.15 inches long and 5.2 inches wide, the following tables of data are given concerning preferred dimensional tolerances in inches:

Table I Final Rod Size gy Rod Per Block Grmgs Block Thickness Table 11 Web Grinding Wheel Final Rod Size Form Thick Width Radius D epth The foregoing may also be applied, with or without slight dimensional variations, to the powder production of slender articles by sintering from other materials, such as pure or complex alloy metal powders, mixtures of metal and non-metal powders and even to the sintering of nonmetal powders such as refractory oxides, metal oxides, ferrites or any other powder materials from which slender articles are produced and which upon sintering tend to cause distortion or warping of the slender article.

"Preferably, we find our invention particularly applicable to the production of liquid phase sintered products produced from a powder mixture containing low and high melting point constituents, wherein the sintering is carried out at a temperature between the melting point of the low melting constituent and the melting point of the high melting constituent. Examples of compositions produced by liquid phase sintering are those comprising high melting point refractory materials selectedfrom the group consisting of tungsten, molybdenum, chromium, tantalum,

columbium, vanadium, titanium, zirconium and hafnium,

, mixtures of two or more of these metals, and carbides,

borides, nitrides and silicides of these metals liquid phase sintered in the presence of a lower melting point matrix metal, such as copper and copper alloys; the iron group metals iron, nickel and cobalt, including their alloys with each other and iron-base, nickel-base and cobalt-base alloys; silver and well known silver-base alloys; and other matrix metals having lower melting points than the refractory materials. Generally, such matrix metals are those having melting points above 700 C. Liquid phase sintering as employed herein is also meant to include production of sintered articles produced by infiltration which during heating tend to lead to distortion of slender shapes.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:.

1. A method for inhibiting warpage in the production of a longitudinal slender shape from powdered ingredients consisting essentially of a mixture of at least one high melting point refractory material and a matrix metal of lower melting point by compressing said ingredients into a slender shape followed by sintering said shape at an elevated temperature between the melting point of the lower melting matrix metal and the melting point of the high melting point refractory material, said method characterized in the steps of forming as a unitary assembly a plurality of compressed slender shapes lying adjacent each other and supportably connected together via a thin severable longitudinal web between each shape, sintering the unitary assembly at said elevated temperature sufiicient to form a dense structure, cooling said sintered assembly, and separating the sintered shapes from each other by severing the connecting webs along their lengths.

2. The method of claim 1, wherein following separation of the sintered shapes any web-flash remaining on each of the shapes is removed and the shape then subjected to a surface cleaning operation.

3. A method for inhibiting warpage in the production of a longitudinal slender shape from powdered ingredients consisting essentially of a mixture of at least one high melting point refractory material and a matrix metal of lower melting point by compressing said ingredients into a slender shape followed by sintering said shape at an elevated temperature between the melting point of the lower melting matrix metal and the melting point of the high melting point refractory material, said method characterized in the steps of forming as a unitary assembly a plurality of compressed slender shapes lying adjacent each other and supportably connected together via a thin breakable web between each shape, each of said shapes having a ratio of length to cross-sectional thickness of at least about 5:1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to thickness of web of at least about 4: 1, sintering the unitary assembly at said elevated temperature under substantially non-oxidizing conditions sufficient to form a dense structure, cooling said sintered assembly, and separating the sintered shapes from each other by breaking the connecting webs along their lengths.

4. A method for inhibiting warpage in the production of a longitudinal slender shape having a cross sectional thickness ranging from about one eighth to about one and one half inches from powdered ingredients consisting essentially of a mixture of at least one high melting point refractory material and a matrix metal of lower melting point by compressing said ingredients into a slender shape followed by sintering said shape at an elevated temperature between the melting point of the lower melting matrix metal and the melting point of the high melting point refractory material, said method characterized in the steps of forming as a unitary assembly a plurality of compressed slender shapes lying adjacent each other and su portably connected together via a thin breakable web between each shape, each of said shapes having a ratio of length to cross-sectional thickness of at least about 5 :1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to thickness of web of at least about 4:1, sintering the unitary assembly at said elevated temperature under substantially nonoxidizing conditions sufiicient to form a dense structure, cooling said sintered assembly, separating the sintered shapes from each other by breaking the connecting webs along their lengths, removing any web-flash remaining on the slender shapes, and subjecting the sintered shapes to a surface cleaning operation.

5. The method of claim 4, wherein the ratio of web width to web thickness ranges up to about 2:1.

6. A method for inhibiting warpage in the production of a longitudinal slender shape having a cross-sectional thickness ranging from about one eighth to about one and one half inches from powdered ingredients consisting essentially of a mixture of at least one high melting point refractory material and a matrix metal of lower melting point by compressing said ingredients into a slender shape followed by sintering said shape at an elevated temperature between the melting point of the lower melting matrix metal and the melting point of the high melting point refractory material, said method characterized in the steps of forming as a unitary assembly a plurality of compressed slender shapes lying adjacent each other and supportably connected together via a thin breakable web between each shape, each of said shapes having a ratio of length to cross-sectional thickness ranging from about 5:1 to 50:1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to thickness of web ranging from about 4:1 to 30:1, sintering the unitary assembly at said elevated temperature under substantially non-oxidizing conditions sufllcient to form a dense structure, cooling said sintered assembly, separating the sintered shapes from each other by breaking the connecting webs along their lengths, removing any web-flash remaining on the slender shapes, and subjecting the sintered shapes to a surface cleaning operation.

7. A method for inhibiting warpage in the production of longitudinal slender shapes from powdered ingredients by compressing a mixture of ingredients consisting, essentially of at least one high melting point refractory material and at least one matrix metal of lower melting point into a slender shape followed by sintering said shape at an elevated temperature below the melting point of the high melting refractory material but above the melting point of the lower melting matrix metal which comprises, forming as a unitary assembly a plurality of slender shapes lying adjacent each other from said mixed ingredients and supportably connected to each other via a thin breakable web between each shape, each of said shapes having a ratio of length to cross-sectional thickness of at least 1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to web thickness of at least about 4:1, sintering the unitary assembly at said elevated temperature below the melting point of the high melting refractory material but above the melting point of the low melting matrix metal, cooling said sintered assembly, and separating the sintered shapes making up the assembly from each other by breaking the connecting webs along their lengths.

8. A method of inhibiting warpage in the production of longitudinal slender shapes from powdered ingredients by compressing a mixture of ingredients consisting essentially of at least one high melting point refractory material and at least one matrix metal of lower melting point into a slender shape followed by sintering said shape at an elevated temperature below the melting point of the high melting refractory material but above the melting point of the lower melting matrix metal which comprises, forming a compressed slab from said mixed ingredients, machining into said slab a plurality of similar slender shapes lying adjacent each other and supportably connected to each other via a thin breakable web between each shape, each of said shapes having a ratio of length to cross-sectional thickness of at least 5:1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to web thickness of at least about 4:1, sintering the machined slab at said elevated temperature below the melting point of the high melting refractory material but above the melting point of the low melting matrix metal, cooling said machined slab, separating the sintered shapes from each other by breaking the connecting webs along their lengths, removing any Web-flash remaining on the slender shapes, and subjecting said shapes to a surface cleaning operation.

9. The method of claim 8, wherein the ratio of web width to web thickness ranges up to about 2:1.

10. A method of inhibiting warpage in the production of longitudinal slender shapes having a cross sectional thickness ranging from about one-eighth to about one and 3 one half inches from powdered ingredients by compressing a mixture of ingredients consisting essentially of at least one high melting point refractory material and at least one matrix metal of lower melting point into a slender shape followed by sintering said shape at an elevated temperature below the melting point of the high melting refractory material but above the melting point of the lower melting matrix metal which comprises, forming a compressed slab from said mixed ingredients, machining into said slab a plurality of similar slender shapes lying adjacent each other and supportably connected to each other via a thin breakable web between each shape, each of said shapes having a ratio of length to crosssectional thickness ranging from about 5:1 to 50: 1, a web thickness ranging from about 0.025 to 0.075 inch and a ratio of thickness of cross section to Web thickness ranging from about 4:1 to 30:1, sintering the machined slab at said elevated temperature below the melting point of the high melting refractory material but above the melting point of the low melting matrix metal, cooling said machined slab, separating the sintered shapes from each other by breaking the connecting webs along their lengths, removing any web-flash remaining on the slender shapes, and subjecting said shapes to a surface cleaning operation.

11. The method of claim 10 wherein the ratio of web width to web thickness ranges up to about 2:1.

12. A method for inhibiting warpage in the production of a longitudinal slender shape from powdered ingredients consisting essentially of a mixture of a refractory carbide comprising titanium carbide and a lower melting matrix metal comprising a heat treatable steel by compressing said ingredients into a slender shape followed by sintering said shape at an elevated temperature between the melting point of the lower melting matrix metal and the melting point of said refractory carbide, characterized in the steps of forming as a unitary assembly a plurality of compressed slender shapes lying adjacent each other and supportably connected together via a thin breakable longitudinal Web between each shape, sinteringthe unitary assembly at said elevated temperature sufiicient to form a dense structure, cooling said sintered assembly, and separating the sintered shapes from each other by breaking the connecting webs along their lengths.

13. The method of claim 12, wherein following separation of the sintered shapes any web-flash remaining on .each of the shapes is removed and the shape then subjected to a surface cleaning operation.

References Cited in the file of this patent UNITED STATES PATENTS 2,447,437 Smith Aug. 17, 1948 2,838,452 West et al. June 10, 1958 FOREIGN PATENTS 874,129 Great Britain Aug. 2, 1961 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. II, Interscience Publishers, Inc., New York, 1950, page 289. Copy in Patent Qfiice Scientific Library, Tn 695.66. 

1. A METHOD FOR INHIBITING WRAPAGE IN THE PRODUCTION OF A LONGITUDINAL SLENDER SHAPE FROM POWDERED INGREDIENTS CONSISTING ESSENTIALLY OF A MIXTURE OF AT LEAST ONE HIGH MELTING POINT REFRACTORY MATERIAL AND A MATRIX META OF LOWER MELTING POINT BY COMPRESSING SAID INGREDIENTS INTO A SLENDER SHAPE FOLLOWED BY SINTERING SAID SHAPE AT AN ELEVATED TEMPERATURE BETWEEN THE MELTING POINT OF THE LOWER MELTING MATRIX METAL AND THE MELTING POINT OF THE HIGH MELTING POINT REFRACTORY MATERIAL, SAID METHOD CHARACTERIZED IN THE STEPS OF FORMING AS A UNITAY ASSEMBLY A PLURALITY OF COMPRESSED SLENDER SHAPES LYING ADJACENT EACH OTHER AND SUPPORTABLY CONNECTED TOGETHER VIA A THIN SEVERABLE LONGITUDINAL WEB BETWEEN EACH SHAPE, SINTERING THE UNITARY ASSEMBLY AT SAID ELEVATED TEMPERATURE SUFFICIENT TO FORM A DENSE STRUCTURE, COOLING SAID SINTERED ASSEMBLY, AND SEPARATING THE SINTERED SHAPES FROM EACH OTHER BY SEVERING THE CONNECTING WEBS ALONG THEIR LENGTHS. 